This invention relates to an apparatus for explosively forming a tube-tube sheet joint and, more particularly, to such an apparatus utilizing an explosive extending within the tube to be expanded.
Many current designs of heat exchangers feature the use of a plurality of heat exchange tubes disposed within a tube sheet and adapted to receive a primary fluid which is passed through the tubes in a heat exchange relationship with a secondary fluid passing over the tubes.
Various techniques have evolved for securing the tubes within the tube sheet. For example, the tubes have been mechanically expanded into the tube sheets utilizing a mandrel or the like, or by rolling, i.e., by applying an outward radial force against the entire surface of the tubes. However, these techniques enjoy several disadvantages including local metal deformation, general lengthening of the tubes, and the application of axial stresses on the tube weld and compressive strains in the tube wall.
In order to overcome the foregoing disadvantages, a technique of explosively forming the tubes within the tube sheet has evolved. According to this technique, an explosive charge is disposed within the area of overlap between the tubes and the tube sheet and is surrounded by a force transmitting member which, upon detonation of the explosive charge, expands the tube uniformly against the inner wall of the tube sheet.
One of the most popular explosives for use in this type of environment is generally known as detonating cord and is actually a high explosive fuse which is comprised of a woven fabric tube reinforced with wrappings of fiber and plastic or metal, and may be impregnated with asphalt and wax, and filled with a core of high explosive such as pentaerythritol tetranitrate (PETN). This explosive fuse is sufficient to adequately expand the tubes within the tube sheets in this type of application but enjoys the disadvantage of generating a relatively large amount of debris and gases. Since the common practice is to extend the explosive fuse externally of the tubes and to an externally-located detonator cap or the like, it can be appreciated that the foregoing debris and gases are unacceptable in many industrial applications, not the least of which is in connection with nuclear heat exchanger explosive expansion work.